Crime-scene tape cordons off the Santa Barbara cul-de-sac where Cheri Domingo and Gregory Sanchez were murdered in 1981. Thirty years later, DNA connected the double homicide to the Golden State Killer.
Photograph: Santa Barbara County Sheriff’s Office / Orange County Sheriff’s Department

For at least a decade between 1976 and 1986, a psychopath stalked California. He targeted bungalows in middle-class neighbourhoods stretching from Sacramento in the north to Dana Point, nearly 450 miles to the south. He wore a mask. He was white, probably in his late teens or 20s, wore size nine shoes and had type A blood. He sometimes stuttered, and sometimes cried after attacking his victims. He had a small penis.

This is almost all that is known about the prolific rapist and murderer who has been variously dubbed the Original Night Stalker, the East Area Rapist and, perhaps most evocatively, the Golden State Killer. This last epithet was coined by the late Michelle McNamara, whose posthumous book chronicles her decade-long quest to identify this mysterious bogeyman. Like the Zodiac Killer, who terrorised California in the late 60s, the Golden State Killer was never apprehended, and his case continues to intrigue amateur sleuths.

The FBI’s Golden State Killer wanted poster, issued in 2016 with a $50,000 reward for information on the unknown suspect Photograph: FBI

A lifelong devotee of true crime, McNamara blogged about her DIY cold-case investigations on the website truecrimediary.com. In 2007, she learned of the East Area Rapist, and her life seems to have changed. “There’s a scream lodged permanently in my throat now,” she writes in her New York Times bestselling, unforgettable I’ll Be Gone in the Dark. When she died at the age of 46 in 2016, leaving the book unfinished, she had amassed 3,500 files related to the case, plus dozens of notebooks, legal pads, digitised police reports and 37 boxes from an Orange County prosecutor (the book was finished by her lead researcher and a colleague). Some of this material went into the blockbuster story she wrote about the case for Los Angeles magazine in 2013, but this book is the real testament to how all-consuming and dogged McNamara’s search was.

Like other recent true-crime books – Claudia Rowe’s The Spider and the Fly, Carolyn Murnick’s The Hot One – McNamara’s is as much a memoir as it is a procedural. Early chapters describe the unsolved murder of 24-year-old Kathleen Lombardo in 1984, which occurred just steps from McNamara’s childhood home in Oak Park, Illinois. The case fascinated the teenage McNamara and whetted her appetite for the dark side. “I was a hoarder of ominous and puzzling details,” she writes. “I developed a Pavlovian response to the word ‘mystery’. My library record was a bibliography of the macabre and true. When I meet people and hear where they’re from I orient them in my mind by the nearest unsolved crime.”

As a record of obsession, I’ll Be Gone in the Dark delivers a nearly fluorescent portrait of the fanatic’s life: the sleepless nights and shut-in days, the rabbit holes of online message boards, the underground economies of samizdat information. In one vivid passage, McNamara recounts holing up in a Sacramento hotel room to review 4,000 pages of police reports on a flash drive she’d just acquired. A raucous wedding reception was being held 10 floors below. “I was jittery from sugar, hunger, and spending too much time alone in the dark absorbing a 50-chapter horror story narrated in the kind of dead voice used by desk clerks at the DMV [Department of Motor Vehicles],” McNamara writes. “My eyes were stripped by computer glare and as devoid of moisture as if they’d been vacuumed clean by an airplane toilet. Kool & the Gang’s ‘Celebration’ wasn’t the soundtrack for my frame of mind.”

Some victims described how he cried after attacking them; one said he ate crackers in the kitchen during the assault

This excerpt indicates what’s so unique and convivial about the book: no matter how grisly things get – and there’s no shortage of horror with more than 50 sexual assaults, at least 10 murders, and tableaux of psychological torture – McNamara retains a sense of humour. But it’s a humour tempered by moral exigency. To identify a killer is to take away his power and render him banal, McNamara argues. In one of the book’s many sharp insights, she likens herself and all amateur detectives to the killers they seek. Both perpetrator and sleuth share an uncommon and singular compulsion. One seeks to destroy, while the other seeks to create, however haphazardly, some kind of explanation.

Explanations are hard to come by in the case of the Golden State Killer. His first crime was probably a rape in Rancho Cordova, a suburb of Sacramento, in June 1976. His first murder occurred two years later. All the evidence and eyewitness accounts add up to a mere fragment, a faceless cipher who taunted the police and his victims with crank phone calls, kept houses under surveillance before his attacks and seemed to relish playing mind games. He bound some of his female victims, raped them, and then went silent. Just when they believed the terror was over, he’d whisper in their ears or scrape a knife along their backs. He told one victim that he’d been in the army; others described how he cried or hyperventilated after attacking them. According to one, he took a break from his assault to eat crackers in the kitchen. One couple told detectives he “seemed like someone straining to appear tough”.

Investigators process the Rancho Cordova backyard where two victims, Brian and Katie Maggiore, were found shot dead in 1978. Photograph: Santa Barbara County Sheriff’s Office / Orange County Sheriff’s Department

Whatever the extent of his instability, his crimes were real, and they spread panic in California. By 1977, he averaged two rapes a month. McNamara describes how entire towns in northern California were transformed into de facto garrisons patrolled by vigilante squads. “In one house, tambourines were tied to every door and window,” she writes. “Hammers went under pillows. Nearly 3,000 guns were sold in Sacramento County between January and May. Many people refused to sleep between 1 and 4am. Some couples slept in shifts, one of them always stationed on the living room couch, a rifle pointed at the window.”

The killer was never caught or even identified. Decades after retirement, some detectives are unable to shake the case

McNamara resurrects two Californian offenders from the 1970s who have largely faded into obscurity – the Early Bird Rapist and the Ransacker. Nothing connects these men. As McNamara points out, crime rates were high all across the country in the 70s. It was a decade adrift from violence and the nihilism of seeing the counterculture erode into drugs, anomie and post-Vietnam malaise. California metabolised 60s idealism and 70s cynicism into a kind of toxic slag. Some of McNamara’s most arresting passages detail the copycat strip malls and subdivisions that replaced orange groves and lush farmland.

This hangover feeling is part of the larger tragedy that the book documents. More than 8,000 suspects were investigated as part of the Golden State Killer case. Detectives crisscrossed the country to retrieve DNA samples and follow leads. Yet the killer was never caught or even identified. Even decades after retirement, some detectives are unable to shake the case. “The Golden State Killer haunts their dreams,” McNamara writes. “He’s ruined their marriages. He’s burrowed so deeply inside their heads that they want to, or have to, believe that if they locked eyes with him, they’d know.”

You come away from I’ll Be Gone in the Dark suspecting much the same of McNamara. In the book’s lyrical epilogue, she addresses the killer, who she imagines is now an old man somewhere in the dregs of America. (She may be right. In 2001, a man presumed to be him called a woman he’d assaulted 24 years earlier: “Remember when we played?” he whispered.) McNamara sketches a hypothetical but hopeful scene in which a car pulls up to the kerb and detectives emerge to finally arrest the monster who has eluded them for more than 40 years. You can’t help but believe that had McNamara lived, that outcome might have been a little more likely.

• I’ll Be Gone in the Dark: One Woman’s Obsessive Search for the Golden State Killer is published by Faber. To order a copy for £11.04 (RRP £12.99) go to guardianbookshop.com or call 0330 333 6846. Free UK p&p over £10, online orders only. Phone orders min p&p of £1.99..